FIG. 1 illustrates a general magnetic resonance imaging (MRI) system, which is disclosed in U.S. Pat. No. 7,002,347. For convenience of explanation, terms and reference numerals are modified. An MRI system may include a magnet 1 for generating a main magnetic field, a transmitting radio frequency (RF) coil 2 for generating a magnetic field that makes hydrogen nucleuses of a test object 4 enter an excited state, a receiving RF coil 3 for receiving an RF signal generated when the excited hydrogen nucleuses of the test object 4 returns to a ground state, and a table 5 for placing the test object 4 thereon. In general, the main magnetic field is referred to as a B0 magnetic field and a magnetic field generated by the transmitting RF coil 2 is referred to as a B1 magnetic field. The B0 magnetic field is formed in a Z-axis direction and the B1 magnetic field is formed in an X-axis direction perpendicular to the direction of the B0 magnetic field. The transmitting RF coil 2 not only generates a magnetic field, but also receives an RF signal. Also, the receiving RF coil 3 not only receives the RF signal, but also generates the B1 magnetic field, if necessary. Accordingly, unless specified otherwise below, the term “RF coil” is used as having both meanings of a transmitting RF coil and a receiving RF coil. The strength and uniformity of the B0 magnetic field are important in relation to the quality of an MRI image. Also, uniformity of the B1 magnetic field is important in relation to the quality of an MRI image. Among them, regarding the B1 magnetic field generated by the transmitting RF coil 2, even when the transmitting RF coil 2 generates a uniform B1 magnetic field, the B1 magnetic field becomes irregular on the test object 4 to be measured due to a varying distance between the transmitting RF coil 2 and the test object 4.
FIG. 2 illustrates that a B1 magnetic field generated by an RF coil 6 becomes irregular on a test object due to a varying distance between the RF coil 6 and the test object.
Test objects 7 and 8 are each placed between a pair of RF coils 6. When the shape of the test object 7 is rectangular, a distance L between the RF coil 6 and the test object 7 is constant as indicated by an arrow. However, when the shape of the test object 8 is circular, the distance between the RF coil 6 and the test object 8 varies between distances L1 and L2 depending on a position on a surface of the test object 8 as indicated by arrows. As is well known to one of ordinary skill in the art, as the distance from the RF coil 6 increases, the strength of the B1 magnetic field decreases. Accordingly, the strength of the B1 magnetic field generated by the RF coil 6 is uniform at any position on the test object 7 having the constant distance L. However, the strength of the B1 magnetic field on the test object 8 having different distances L1 and L2 varies depending on the distance between the test object 8 and the RF coil 6. Since the strength of the B1 magnetic field differs at positions having a varying distance between the RF coil 6 and the test object 8, the level of excited state of hydrogen nucleuses varies accordingly and thus, the RF signal of hydrogen nucleuses used to form an MRI image of the test object differs, which deteriorates the quality of the MRI image. Even when the distance from the test object 7 is constant, the B1 magnetic field may be irregular due to a problem of the RF coil. In this case, the quality of the MRI image is also deteriorated.
To address the above problem, a variety of solutions have been developed.
Related arts to improve uniformity of the B1 magnetic field may include U.S. Pat. Nos. 5,017,872, 7,002,347, 7,242,192, and 8,188,737.
FIG. 3 illustrates an example of making a B1 magnetic field uniform, which is disclosed in U.S. Pat. No. 5,017,872. For convenience of explanation, terms and reference numerals are modified.
An RF coil structure 9 may include an RF coil 10, an RF shield 11, and a high dielectric material 12. When a test object 13 is placed inside the RF coil structure 9, the B1 magnetic field becomes irregular. To address the irregularity problem, the high dielectric material 12 fills a space between the RF coil 10 and the RF shield 11.
FIG. 4 illustrates an example of making a B1 magnetic field uniform, which is disclosed in U.S. Pat. No. 7,242,192. For convenience of explanation, terms and reference numerals are modified.
An RF coil structure 14 may include a main RF coil 15 and an auxiliary RF coil 16. Although not illustrated in the drawing, when the RF coil structure 14 is in use, a distance between a test object and the RF coil structure 14 is not constant and thus the B1 magnetic field is not uniform. To address this problem, the auxiliary RF coil 16 is arranged to make the B1 magnetic field uniform.
However, among the above-described structures used to make the B1 magnetic field uniform, in the structure of FIG. 3, since the high dielectric material 12 is uniformly arranged between the RF shield 11 and the RF coil 10, it may be difficult to overcome an influence of the difference in the distance between the test object and the RF coil (to be described in relation to FIG. 5). Also, in the structure of FIG. 4, it is inconvenient to use the auxiliary RF coil 15, in addition to the main RF coil 15.